262 PETER SPENCE 



years had developed until the one now under 

 consideration appeared, which was in every 

 way very much more complete. 



Shale from the coal measures was the raw 

 material which he principally relied on. After 

 being calcined in large heaps over flues, the 

 burnt scales were cast, warm, into leaden 

 tanks, in which there was sulphuric acid ; the 

 shales in time absorbed the acid and became 

 quite dry. The vat was then emptied of the 

 saturated shales, which were put into another 

 vat and covered with boiling mother liquor, 

 which had been neutralised with ammonia 

 vapour; the sulphate of alumina was hereby 

 dissolved out, and crystallised alum obtained. 

 If it was found that the sulphate of alumina 

 was not sufficiently soluble, that the shales 

 had not been sufficiently acted upon by the 

 sulphuric acid, the saturated shales were 

 heated in the furnace and then put into the 

 dissolving vat. But one point Spence kept 

 in view was that no expense must be incurred 

 by any process of evaporation prior to 

 crystallisation u the clear solutions of sulphate 

 of alumina must be of sufficient strength for 

 the crystallisation of alum without evapora- 

 tion." 



The same patent specifies that the mother 



